1. Field of the Invention
This invention pertains to apparatus for pumping liquids, and more particularly to apparatus for positively pumping liquid and viscous liquid food products.
2. Description of the Prior Art
Positive displacement pumps for pumping liquid food products of various viscosities are well known. For example, catalog number PR73 published by the Ladish Co., Tri-Clover Division, Kenosha, Wis., describes positive displacement rotary pumps capable of pumping both high and low viscosity consumable liquids.
A primary requirement of the food processing industry is that all apparatus must meet rigid sanitation standards. U.S. Pat. No. 3,095,203 illustrates one design for sealing a liquid food product from possible sources of contamination within a pump. Sanitation requirements dictate, to a large extent, the design of food handling pumping equipment. Unlike pumps for handling non-edible liquids, as for example, hydraulic oil, sanitation pumps do not have bearings outboard of the pump impeller. Such bearings are not feasible because of inherent problems with lubrication, seal requirements and bearing materials. In addition, sanitary pump users demand pumps that are designed to be disassembled, cleaned and reassembled with a minimum of effort and down time. U.S. Pat. No. 3,227,088 discloses means for retaining the components of a pump as a unit during operation, but which allows quick and easy disassembly for cleaning.
The lack of outboard bearings on the impeller shaft makes shaft deflection a critical factor in the design and operation of sanitary pumps. As discharge pressures increase, the shaft deflection also increases. Discharge pressures in a typical well-known pump are limited to about 50 psig to 70 psig. Higher pressures result in reduced internal clearances to the point of interference between the rotors or impellers (hereinafter called impellers) and the pump housing. The consequence is that the tips of the impellers wear, which increases clearance with the housing, reduces pump efficiency and increases noise and vibration. Also, the abraded particles may be a source of contamination to the food product.
A related wear problem is involved in the mounting of the pump impeller to the impeller shaft. For ease of assembly and disasssembly, the impeller typically is driven by and is located on splines machined into the shaft. Due to normal manufacturing tolerances, a splined impeller inherently possesses a certain amount of looseness with respect to the shaft. The looseness is detrimental in that the impeller may cock slightly on the shaft splines, causing the impeller lobe tips to contact the housing, resulting in wear.
In sanitation pumps, problems arise in axially securing the impeller to the impeller shaft because of two conflicting requirements. On the one hand, it is necessary to firmly secure the impeller to the shaft. On the other hand, the impeller must be quickly and easily removable from the shaft for cleaning. One common design is to thread a single lock nut onto the shaft and against the impeller. This design has not proven completely satisfactory. Pumps are reversible, and the nut has a tendency to loosen and even fall off the end of the shaft. To prevent the loosened nut from damaging the shaft and pump, a clearance space large enough to hold the nut must be provided around the end of the shaft. A jam nut in conjunction with a lock nut, although somewhat superior to the single nut concept, has also proven unsatisfactory, primarily because of the reversible nature of the pump. In fact, the two nut design requires a clearance space twice as large as with a single nut. If this space is not present to afford spinoff, the loosened nuts can wedge in the cover and cause considerable damage to the pump. Another problem is that workmen cleaning the pump tend to place the nuts on their faces on any convenient surface. The result is that the faces, which must be flat and smooth to mate properly, become nicked. Consequently, the holding force between two abutting nuts diminishes to the point of eventual ineffectiveness. Polishing the nicked faces is not feasible because of the difficulty of maintaining perpendicularity between the nut axis and the nut faces.
Accordingly, a need exists for a food processing pump that can be operated at high pressures without wear caused by pump deflection and that includes components that consistently lock securely together but that can be quickly and easily disassembled.